Shale is a fine-grained,
clastic sedimentary rock
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particle ...
formed from
mud that is a mix of flakes of
clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin,
Al2 Si2 O5(
OH)
4) and tiny fragments (
silt
Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel wh ...
-sized particles) of other minerals, especially
quartz and
calcite.
[Blatt, Harvey and Robert J. Tracy (1996) ''Petrology: Igneous, Sedimentary and Metamorphic'', 2nd ed., Freeman, pp. 281–292 ] Shale is characterized by its tendency to split into thin layers (
laminae) less than one centimeter in thickness. This property is called ''
fissility''.
Shale is the most common sedimentary rock.
The term ''shale'' is sometimes applied more broadly, as essentially a synonym for
mudrock, rather than in the more narrow sense of clay-rich fissile mudrock.
Texture
Shale typically exhibits varying degrees of
fissility. Because of the parallel orientation of
clay mineral flakes in shale, it breaks into thin layers, often splintery and usually parallel to the otherwise indistinguishable
bedding planes
In geology, a bed is a layer of sediment, sedimentary rock, or pyroclastic material "bounded above and below by more or less well-defined bedding surfaces".Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds., 2005. ''Glossary of Geology'' ...
.
[Blatt, Harvey and Robert J. Tracy (1996) ''Petrology: Igneous, Sedimentary and Metamorphic'', 2nd ed., Freeman, pp. 281–292 ] Non-fissile
rocks
In geology, rock (or stone) is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its chemical composition, and the way in which it is formed. Rocks form the Earth's ...
of similar composition and particle size (less than 0.0625 mm) are described as
mudstones (1/3 to 2/3 silt particles) or
claystones (less than 1/3 silt). Rocks with similar particle sizes but with less clay (greater than 2/3 silt) and therefore grittier are
siltstones.
[
]
Composition and color
Shales are typically gray in color and are composed of clay minerals and quartz grains. The addition of variable amounts of minor constituents alters the color of the rock. Red, brown and green colors are indicative of ferric oxide (hematite
Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of ...
– reds), iron hydroxide ( goethite – browns and limonite – yellow), or micaceous minerals ( chlorite, biotite and illite – greens).[ The color shifts from reddish to greenish as ]iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
in the oxidized ( ferric) state is converted to iron in the reduced ( ferrous) state. Black shale results from the presence of greater than one percent carbonaceous material and indicates a reducing environment.[ Pale blue to blue-green shales typically are rich in carbonate minerals.
Clays are the major constituent of shales and other mudrocks. The clay minerals represented are largely kaolinite, montmorillonite and illite. Clay minerals of Late ]Tertiary
Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago.
The period began with the demise of the non- avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...
mudstones are expandable smectites, whereas in older rocks (especially in mid-to early Paleozoic shales) illites predominate. The transformation of smectite to illite produces silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is o ...
, sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
, calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...
, magnesium, iron and water. These released elements form authigenic quartz, chert, calcite, dolomite, ankerite, hematite and albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula . It is a tectosilica ...
, all trace to minor (except quartz) minerals found in shales and other mudrocks.[ A typical shale is composed of about 58% clay minerals, 28% quartz, 6% ]feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feld ...
, 5% carbonate minerals, and 2% iron oxides. Most of the quartz is detrital (part of the original sediments that formed the shale) rather than authigenic (crystallized within the shale after deposition).
Shales and other mudrocks contain roughly 95 percent of the organic matter in all sedimentary rocks. However, this amounts to less than one percent by mass in an average shale. Black shales, which form in anoxic conditions, contain reduced free carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes ...
along with ferrous iron (Fe2+) and sulfur (S2−). Amorphous
In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal.
Etymology
The term comes from the Greek language ...
iron sulfide, along with carbon, produce the black coloration.[ Because amorphous iron sulfide gradually converts to ]pyrite
The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral.
Pyrite's metallic luster and pale brass-yellow hue giv ...
, which is not an important pigment, young shales may be quite dark from their iron sulfide content, in spite of a modest carbon content (less than 1%), while a black color in an ancient shale indicates a high carbon content.
Most shales are marine in origin, and the groundwater
Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidat ...
in shale formations is often highly saline
Saline may refer to:
* Saline (medicine), a liquid with salt content to match the human body
* Saline water, non-medicinal salt water
* Saline, a historical term (especially US) for a salt works or saltern
Places
* Saline, Calvados, a commune in ...
. There is evidence that shale acts as a semipermeable medium, allowing water to pass through while retaining dissolved salts.
Formation
The fine particles that compose shale can remain suspended in water long after the larger particles of sand have been deposited. As a result, shales are typically deposited in very slow moving water and are often found in lakes and lagoon
A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into '' coastal lagoons'' (or ''barrier lagoons' ...
al deposits, in river deltas, on floodplains and offshore below the wave base. Thick deposits of shale are found near ancient continental margins
A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margin ...
and foreland basins
Foreland may refer to:
* a landform projecting into the sea, such as a headland or a promontory
* an area of land in front of something
**Foreland basin, in geology, the zone that receives sediment from an adjacent mountain chain
**Glacier foreland ...
. Some of the most widespread shale formations were deposited by epicontinental sea
An inland sea (also known as an epeiric sea or an epicontinental sea) is a continental body of water which is very large and is either completely surrounded by dry land or connected to an ocean by a river, strait, or "arm of the sea". An inland se ...
s. Black shales are common in Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
strata on the margins of the Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth#Surface, Earth's surface and about 29% of its water surface area. It is known to separate the "Old World" of Afr ...
, where they were deposited in fault-bounded silled basins associated with the opening of the Atlantic during the breakup of Pangea. These basins were anoxic, in part because of restricted circulation in the narrow Atlantic, and in part because the very warm Cretaceous seas lacked the circulation of cold bottom water that oxygenates the deep oceans today.
Most clay must be deposited as aggregates and floccules, since the settling rate of individual clay particles is extremely slow. Flocculation
Flocculation, in the field of chemistry, is a process by which colloidal particles come out of suspension to sediment under the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The action differs from ...
is very rapid once the clay encounters highly saline sea water. Whereas individual clay particles are less than 4 microns in size, the clumps of clay particles produced by flocculation vary in size from a few tens of microns to over 700 microns in diameter. The floccules start out water-rich, but much of the water is expelled from the floccules as the clay minerals bind more tightly together over time (a process called syneresis). Clay pelletization by organisms that filter feed is important where flocculation is inhibited. Filter feeders produce an estimated 12 metric tons of clay pellets per square kilometer per year along the U.S. Gulf Coast
The Gulf Coast of the United States, also known as the Gulf South, is the coastline along the Southern United States where they meet the Gulf of Mexico. The coastal states that have a shoreline on the Gulf of Mexico are Texas, Louisiana, Mississ ...
.
As sediments continue to accumulate, the older, more deeply buried sediments begin to undergo diagenesis. This mostly consists of compaction
Compaction may refer to:
* Soil compaction, for mechanically induced compaction near the ground surface
* Compaction of ceramic powders
* Compaction (geology), part of the process of lithification involving mechanical dewatering of a sediment by ...
and lithification
Lithification (from the Ancient Greek word ''lithos'' meaning 'rock' and the Latin-derived suffix ''-ific'') is the process in which sediments compact under pressure, expel connate fluids, and gradually become solid rock. Essentially, lithificatio ...
of the clay and silt particles. Early stages of diagenesis, described as ''eogenesis'', take place at shallow depths (a few tens of meters) and are characterized by bioturbation and mineralogical changes in the sediments, with only slight compaction. Pyrite
The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral.
Pyrite's metallic luster and pale brass-yellow hue giv ...
may be formed in anoxic mud at this stage of diagenesis.
Deeper burial is accompanied by ''mesogenesis'', during which most of the compaction and lithification takes place. As the sediments come under increasing pressure from overlying sediments, sediment grains move into more compact arrangements, ductile grains (such as clay mineral grains) are deformed, and pore space is reduced. In addition to this physical compaction, chemical compaction may take place via pressure solution. Points of contact between grains are under the greatest strain, and the strained mineral is more soluble than the rest of the grain. As a result, the contact points are dissolved away, allowing the grains to come into closer contact.
It is during compaction that shale develops its fissility, likely through mechanical compaction of the original open framework of clay particles. The particles become strongly oriented into parallel layers that give the shale its distinctive fabric. Fissility likely develops early in the compaction process, at relatively shallow depth, since fissility does not seem to vary with depth in thick formations. Kaolinite flakes have less tendency to align in parallel layers than other clays, so kaolinite-rich clay is more likely to form nonfissile mudstone than shale. On the other hand, black shales often have very pronounced fissility (''paper shales'') due to binding of hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...
molecules to the faces of the clay particles, which weakens the binding between particles.
Lithification follows closely on compaction, as increased temperatures at depth hasten deposition of cement
A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement m ...
that binds the grains together. Pressure solution contributes to cementing, as the mineral dissolved from strained contact points is redeposited in the unstrained pore spaces. The clay minerals may be altered as well. For example, smectite is altered to illite at temperatures of about , releasing water in the process. Other alteration reactions include the alteration of smectite to chlorite and of kaolinite to illite at temperatures between . Because of these reactions, illite composes 80% of Precambrian shales, versus about 25% of young shales.
Unroofing of buried shale is accompanied by ''telogenesis'', the third and final stage of diagenesis. As erosion reduces the depth of burial, renewed exposure to meteoric water produces additional changes to the shale, such as dissolution of some of the cement to produce secondary porosity. Pyrite may be oxidized to produce gypsum
Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and dr ...
.
''Black shales'' are dark, as a result of being especially rich in unoxidized carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes ...
. Common in some Paleozoic and Mesozoic
The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Creta ...
strata, black shales were deposited in anoxic, reducing environments, such as in stagnant water columns. Some black shales contain abundant heavy metals such as molybdenum, uranium, vanadium, and zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic t ...
. The enriched values are of controversial origin, having been alternatively attributed to input from hydrothermal fluids during or after sedimentation or to slow accumulation from sea water over long periods of sedimentation.
File:Shale in Potokgraben.jpg, Shale in Potokgraben, the Karawanks, Austria
Austria, , bar, Östareich officially the Republic of Austria, is a country in the southern part of Central Europe, lying in the Eastern Alps. It is a federation of nine states, one of which is the capital, Vienna, the most populous ...
File:MesselShaleSplitting.JPG, Splitting shale ( Messel oil shale) with a large knife to reveal fossils
File:Shale 8040.jpg, Weathering
Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs '' in situ'' (on site, with little or no movemen ...
shale at a road cut in southeastern Kentucky
Kentucky ( , ), officially the Commonwealth of Kentucky, is a state in the Southeastern region of the United States and one of the states of the Upper South. It borders Illinois, Indiana, and Ohio to the north; West Virginia and Virgini ...
Fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
s, animal tracks or burrows and even raindrop impressions are sometimes preserved on shale bedding surfaces. Shales may also contain concretions consisting of pyrite, apatite
Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common ...
, or various carbonate minerals.
Shales that are subject to heat and pressure of metamorphism alter into a hard, fissile, metamorphic rock known as slate. With continued increase in metamorphic grade the sequence is phyllite, then schist and finally gneiss.
As hydrocarbon source rock
Shale is the most common source rock for hydrocarbons (natural gas
Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon ...
and petroleum
Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crude ...
). The lack of coarse sediments in most shale beds reflects the absence of strong currents in the waters of the depositional basin. These might have oxygenated the waters and destroyed organic matter before it could accumulate. The absence of carbonate rock in shale beds reflects the absence of organisms that might have secreted carbonate skeletons, also likely due to an anoxic environment. As a result, about 95% of organic matter in sedimentary rocks is found in shales and other mudrocks. Individual shale beds typically have an organic matter content of about 1%, but the richest source rocks may contain as much as 40% organic matter.
The organic matter in shale is converted over time from the original proteins, polysaccharides
Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with w ...
, lipids, and other organic molecules to kerogen, which at the higher temperatures found at greater depths of burial is further converted to graphite and petroleum.
Historical mining terminology
Before the mid-19th century, the terms slate, shale and schist were not sharply distinguished. In the context of underground coal mining
Coal mining is the process of extracting coal from the ground. Coal is valued for its energy content and since the 1880s has been widely used to generate electricity. Steel and cement industries use coal as a fuel for extraction of iron from ...
, shale was frequently referred to as slate well into the 20th century. Black shale associated with coal seams is called black metal.
See also
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References
External links
{{commonscatinline
Industrial minerals